The invention relates to the technical field of DLC (Diamond-Like Carbon) coatings, especially those for friction parts.
The invention has an especially advantageous application in reducing the coefficient of friction, of piston rods, camshafts, valve lifters, cylinders, piston rings etc. for example and, more generally, in all cases of friction under load. In order to reduce such friction, those skilled in the art are perfectly aware of the possibility of applying a DLC coating to the part or parts in question.
The invention may also have applications where there is a requirement for the surface provided by the coating to have a black colour without this involving any attempt to reduce friction.
It is also generally known by those skilled in the art that the low adhesion of DLC films on components is capable of posing real problems in certain applications. One technical solution in order to improve adhesion involves using a metal-containing adhesive layer based on silicon or chromium for instance. Various technical solutions have been proposed.
Document WO2011/018252, for example, describes a friction part having a coating consisting of an adhesive layer, a metal-containing DLC coating and a metal-free DLC coating. The adhesive layer is preferably a chromium coating having a maximum thickness of 1 μm whereas the metal-containing DLC coating is preferably made of WCC tungsten carbide. The ratios of the thicknesses of the various layers and coatings are limited to certain ranges of values so that, outside these ranges, if the DLC thickness is too small, the service life of the component will be shortened and if the DLC thickness is too large, the component will wear prematurely with risks of peeling or flaking off.
Document WO0179585 discloses a multilayer system having an adhesive layer, a transition layer and a layer of diamond-like carbon. The adhesive layer comprises an element in the 4th, 5th or 6th subgroup and silicon whereas the transition layer comprises carbon and at least one element in the 4th, 5th or 6th subgroup and silicon. The upper layer is predominantly made of diamond-like carbon. The system has a hardness of at least 15 GPa and an adhesive strength of at least HF3.
Generally speaking, delamination of the DLC film from this undercoat associated with internal stresses in the DLC film is observed and this delamination increases the thicker such a film is. It is also apparent that this adhesive undercoat is formed in a separate step and that this increases the cost of the method and makes such a method more complicated.
The invention has set itself the object of overcoming these drawbacks in a simple, dependable, effective and efficient manner.
The problem which the invention intends to solve is to produce DLC films having improved adherence without using a metal-containing adhesive undercoat (e.g. silicon or chromium) based on the teaching of the prior art.